Genetic control of sensitivity to hippocampal cell death induced by kainic acid: A quantitative trait loci analysis

Stroke injury induced by distal middle cerebral artery occlusion is resistant to N-methyl-d-aspartate receptor antagonism in FVB/NJ mice

Although N-methyl-d-aspartate receptor (NMDAR) antagonism has been shown to have a neuroprotective effect in many preclinical stroke models, the efficacy of this antiexcitotoxicity strategy in clinical trials in stroke patients has been disappointing. Interestingly, it has been reported that NMDAR antagonism is not neuroprotective in C57BL/6 mice subjected to distal middle cerebral artery occlusion (dMCAO), supporting the notion that whether these treatments are neuroprotective depends on the type of cerebral ischemia. However, because C57BL/6 mice are inherently resistant to excitotoxicity, the reported lack of neuroprotection could also be explained by the difference in the mouse strain studied rather than the stroke model used. Here we examined the neuroprotective efficacy of NMDAR antagonism in FVB/NJ mice, an excitotoxicity-prone mouse strain, subjected to dMCAO. Although C57BL/6 mice are known to have an excitotoxicity-resistant genetic background and FVB/NJ mice are known to have an excitotoxicity-prone genetic background, the infarct volume and density of neurodegenerating neurons were similar in the two mouse strains following dMCAO. In addition, none of the antiexcitotoxicity agents studied, including the canonical NMDAR antagonist MK801 and the therapeutic peptides Tat-NR2B9c and L-JNKI-1, protected the FVB/NJ mouse brain against ischemic damage induced by dMCAO. In conclusion, our data demonstrated that FVB/NJ mice are no more susceptible to cerebral ischemia than C57BL/6 mice and that NMDAR antagonism is ineffective in mice, even in an excitotoxicity-prone strain, subjected to dMCAO.

Survival and Motor Phenotypes in FVB C9-500 ALS/FTD BAC Transgenic Mice Reproduced by Multiple Labs

Mordes et al. (2020) did not detect the survival or motor phenotypes in C9orf72 BAC transgenic mice originally described by Liu et al. (2016). We discuss methodological differences between the Mordes and Liu studies, several additional studies in which survival and motor phenotypes were found, and possible environmental and genetic effects. First, Nguyen et al. (2020) showed robust ALS/FTD phenotypes in C9-BAC versus non-transgenic (NT) mice and that α-GA₁ treatment improved survival, behavior, and neurodegeneration. The groups of Gelbard and Saxena also show decreased survival of C9-BAC versus NT mice and neuropathological and behavioral deficits similar to those shown by Liu et al. (2016). Although FVB/N mice can have seizures, increases in seizure severity and death of C9 and NT animals, which may mask C9 disease phenotypes, have been observed in recent C9-500 FVB/NJ-bred cohorts. In summary, we provide an update on phenotypes seen in FVB C9-BAC mice and additional details to successfully use this model. This Matters Arising Response paper addresses the Mordes et al. (2020) Matters Arising paper, published concurrently in Neuron.

The relevance of inter- and intrastrain differences in mice and rats and their implications for models of seizures and epilepsy

It is becoming increasingly clear that the genetic background of mice and rats, even in inbred strains, can have a profound influence on measures of seizure susceptibility and epilepsy. These differences can be capitalized upon through genetic mapping studies to reveal genes important for seizures and epilepsy. However, strain background and particularly mixed genetic backgrounds of transgenic animals need careful consideration in both the selection of strains and in the interpretation of results and conclusions. For instance, mice with targeted deletions of genes involved in epilepsy can have profoundly disparate phenotypes depending on the background strain. In this review, we discuss findings related to how this genetic heterogeneity has and can be utilized in the epilepsy field to reveal novel insights into seizures and epilepsy. Moreover, we discuss how caution is needed in regards to rodent strain or even animal vendor choice, and how this can significantly influence seizure and epilepsy parameters in unexpected ways. This is particularly critical in decisions regarding the strain of choice used in generating mice with targeted deletions of genes. Finally, we discuss the role of environment (at vendor and/or laboratory) and epigenetic factors for inter- and intrastrain differences and how such differences can affect the expression of seizures and the animals' performance in behavioral tests that often accompany acute and chronic seizure testing.

Age-Dependent Resistance to Excitotoxicity in Htt CAG140 Mice and the Effect of Strain Background

Mouse strain background can influence vulnerability to excitotoxic neuronal cell death and potentially modulate phenotypes in transgenic mouse models of human disease. Evidence supports a contribution of excitotoxicity to the selective death of medium spiny neurons in Huntington's disease (HD). Here, we assess whether strain differences in excitotoxic vulnerability influence striatal cell death in a knock-in mouse model of HD. Previous studies that evaluated resistance to excitotoxic lesions in several mouse models of HD had variable outcomes. In the present study, we directly compare one model on two different background strains to test the contribution of strain to excitotoxicity-mediated neurodegeneration. Mice of the FVB/N strain, which are highly vulnerable to excitotoxicity, become extremely resistant to quinolinic acid-induced striatal neurodegeneration with age, when carrying a huntingtin (Htt) allele expressing a HD transgene (CAG140). The resistance is much greater than the age-dependent resistance that has been previously reported in YAC128 mice. By 12 months of age, both heterozygous and homozygous FVB.CAG140 mice displayed virtually complete resistance to quinolinic acid-induced striatal neurodegeneration. A similar resistance develops in CAG140 mice on a C57BL/6N background although the effect size is smaller because C57BL/6N mice are already resistant due to genetic background. In a direct comparison with the YAC128 mice, FVB.CAG140 mice have greater resistance. FVB.CAG140 mice are also resistant to neurodegeneration following kainic acid-induced status epilepticus suggesting the existence of a common cellular mechanism that provides protection against multiple types of excitotoxic insult. These findings establish FVB.CAG140 mice as a useful model to investigate the cellular and molecular mechanisms that confer neuroprotection against excitotoxicity.

Susceptibility to seizure-induced excitotoxic cell death is regulated by an epistatic interaction between Chr 18 (Sicd1) and Chr 15 (Sicd2) loci in mice

Seizure-induced cell death is believed to be regulated by multiple genetic components in addition to numerous external factors. We previously defined quantitative trait loci that control susceptibility to seizure-induced cell death in FVB/NJ (susceptible) and C57BL/6J (resistant) mice. Two of these quantitative trait loci assigned to chromosomes 18 (Sicd1) and 15 (Sicd2), control seizure-induced cell death resistance. In this study, through the use of a series of novel congenic strains containing the Sicd1 and Sicd2 congenic strains and different combinations of the Sicd1 or Sicd2 sub region(s), respectively, we defined these genetic interactions. We generated a double congenic strain, which contains the two C57BL/6J differential segments from chromosome 18 and 15, to determine how these two segments interact with one another. Phenotypic comparison between FVB-like littermates and the double congenic FVB.B6-Sicd1/Sicd2 strain identified an additive effect with respect to resistance to seizure-induced excitotoxic cell death. It thus appears that C57BL/6J alleles located on chromosomes 18 and 15 interact epistatically in an additive manner to control the extent of seizure-induced excitotoxic cell death. Three interval-specific congenic lines were developed, in which either segments of C57BL/6J Chr 18 or C57BL/6J Chr 15 were introduced in the FVB/NJ genetic background, and progeny were treated with kainate and examined for the extent of seizure-induced cell death. All of the interval-specific congenic lines exhibited reduced cell death in both area CA3 and the dentate hilus, associated with the C57BL/6J phenotype. These experiments demonstrate functional interactions between Sicd1 and Sicd2 that improve resistance to seizure-induced excitotoxic cell death, validating the critical role played by gene-gene interactions in excitotoxic cell death.

A locus on mouse Ch10 influences susceptibility to limbic seizure severity: fine mapping and in silico candidate gene analysis

Identification of genes contributing to mouse seizure susceptibility can reveal novel genes or pathways that provide insight into human epilepsy. Using mouse chromosome substitution strains and interval-specific congenic strains (ISCS), we previously identified an interval conferring pilocarpine-induced limbic seizure susceptibility on distal mouse chromosome 10 (Ch10). We narrowed the region by generating subcongenics with smaller A/J Ch10 segments on a C57BL/6J (B6) background and tested them with pilocarpine. We also tested pilocarpine-susceptible congenics for 6-Hz ECT (electroconvulsive threshold), another model of limbic seizure susceptibility, to determine whether the susceptibility locus might have a broad effect on neuronal hyperexcitability across more than one mode of limbic seizure induction. The ISCS Line 1, which contained the distal 2.7 Mb segment from A/J (starting at rs29382217), was more susceptible to both pilocarpine and ECT. Line 2, which was a subcongenic of Line 1 (starting at rs13480828), was not susceptible, thus defining a 1.0 Mb critical region that was unique to Line 1. Bioinformatic approaches identified 45 human orthologs within the unique Line 1 susceptibility region, the majority syntenic to human Ch12. Applying an epilepsy network analysis of known and suspected excitability genes and examination of interstrain genomic and brain expression differences revealed novel candidates within the region. These include Stat2, which plays a role in hippocampal GABA receptor expression after status epilepticus, and novel candidates Pan2, Cdk2, Gls2 and Cs, which are involved in neural cell differentiation, cellular remodeling and embryonic development. Our strategy may facilitate discovery of novel human epilepsy genes.

Microarray-assisted fine mapping of quantitative trait loci on chromosome 15 for susceptibility to seizure-induced cell death in mice

Prior studies with crosses of the FVB/NJ (FVB; seizure-induced cell death-susceptible) mouse and the C57BL/6J (B6; seizure-induced cell death-resistant) mouse revealed the presence of a quantitative trait locus (QTL) on chromosome 15 that influenced susceptibility to kainic acid-induced cell death (Sicd2). In an earlier study, we confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced cell death through the creation of the FVB.B6-Sicd2 congenic strain, and created three interval-specific congenic lines (ISCLs) that encompass Sicd2 on chromosome 15 to fine-map this locus. To further localise this Sicd2 QTL, an additional congenic line carrying overlapping intervals of the B6 segment was created (ISCL-4), and compared with the previously created ISCL-1-ISCL-3 and assessed for seizure-induced cell death phenotype. Whereas all of the ISCLs showed reduced cell death associated with the B6 phenotype, ISCL-4, showed the most extensive reduction in seizure-induced cell death throughout all hippocampal subfields. In order to characterise the susceptibility loci on Sicd2 by use of this ISCL and identify compelling candidate genes, we undertook an integrative genomic strategy of comparing exon transcript abundance in the hippocampus of this newly developed chromosome 15 subcongenic line (ISCL-4) and FVB-like littermates. We identified 10 putative candidate genes that are alternatively spliced between the strains and may govern strain-dependent differences in susceptibility to seizure-induced excitotoxic cell death. These results illustrate the importance of identifying transcriptomics variants in expression studies, and implicate novel candidate genes conferring susceptibility to seizure-induced cell death.

Kainic acid-induced neuronal degeneration in hippocampal pyramidal neurons is driven by both intrinsic and extrinsic factors: analysis of FVB/N↔C57BL/6 chimeras

The excitotoxic effects of kainic acid (KA) in the mouse hippocampus is strain dependent. Following KA administration, the large majority of hippocampal pyramidal cells die in the FVB/N (FVB) mouse, while the pyramidal cells of the C57BL/6 (B6) strain are largely spared. We generated aggregation chimeras between the sensitive FVB and the resistant B6 strains to investigate whether intrinsic or extrinsic features of a neuron confer cell vulnerability or resistance to KA. The constitutive expression of transgenic green fluorescence protein (GFP) or β-galactosidase expressed from the ROSA26 locus was used to mark cells in FVB or B6 mice, respectively. These makers enable the identification of cells from each parental genotype while TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling)-staining labeled dying cells. The analysis of the percentage of dying cells in FVB-GFP ↔ B6-ROSA chimeras yielded an intriguing mix of both intrinsic and extrinsic factors in the readout of cell phenotype. Thus, normally resistant B6-ROSA pyramidal neurons demonstrated an increasing sensitivity to KA, in a linear fashion, when the percentage of FVB-GFP cells was increased, either across chimeras or in different regions of the same chimera. However, the death of B6-ROSA pyramidal cells never exceeded ∼70% of the total amount of B6 neurons regardless of the amount of FVB cells in the chimeric hippocampus. In a similar manner, FVB-GFP cells show lower amounts of cell death in chimeras that are colonized by B6-ROSA cells, but again, are never fully rescued. These data indicate that both intrinsic and extrinsic factors modulate the sensitivity of hippocampal pyramidal cells to kainic acid.

The effects of glycemic control on seizures and seizure-induced excitotoxic cell death

Background

Epilepsy is the most common neurological disorder after stroke, affecting more than 50 million persons worldwide. Metabolic disturbances are often associated with epileptic seizures, but the pathogenesis of this relationship is poorly understood. It is known that seizures result in altered glucose metabolism, the reduction of intracellular energy metabolites such as ATP, ADP and phosphocreatine and the accumulation of metabolic intermediates, such as lactate and adenosine. In particular, it has been suggested that the duration and extent of glucose dysregulation may be a predictor of the pathological outcome of status. However, little is known about neither the effects of glycemic control on brain metabolism nor the effects of managing systemic glucose concentrations in epilepsy.

Results

In this study, we examined glycemic modulation of kainate-induced seizure sensitivity and its neuropathological consequences. To investigate the relationship between glycemic modulation, seizure susceptibility and its neuropathological consequences, C57BL/6 mice (excitotoxin cell death resistant) were subjected to hypoglycemia or hyperglycemia, followed by systemic administration of kainic acid to induce seizures. Glycemic modulation resulted in minimal consequences with regard to seizure severity but increased hippocampal pathology, irrespective of whether mice were hypoglycemic or hyperglycemic prior to kainate administration. Moreover, we found that exogenous administration of glucose following kainic acid seizures significantly reduced the extent of hippocampal pathology in FVB/N mice (excitotoxin cell death susceptible) following systemic administration of kainic acid.

Conclusion

These findings demonstrate that modulation of the glycemic index can modify the outcome of brain injury in the kainate model of seizure induction. Moreover, modulation of the glycemic index through glucose rescue greatly diminishes the extent of seizure-induced cell death following kainate administration. Our data support the hypothesis that deficient insulin signaling may represent a critical contributing factor in the susceptibility to seizure-induced cell death and this may be an important therapeutic target.

Similar cation channels mediate protection from cerebellar exitotoxicity by exercise and inheritance

Exercise and inherited factors both affect recovery from stroke and head injury, but the underlying mechanisms and interconnections between them are yet unknown. Here, we report that similar cation channels mediate the protective effect of exercise and specific genetic background in a kainate injection model of cerebellar stroke. Microinjection to the cerebellum of the glutamatergic agonist, kainate, creates glutamatergic excito-toxicity characteristic of focal stroke, head injury or alcoholism. Inherited protection and prior exercise were both accompanied by higher cerebellar expression levels of the Kir6.1 ATP-dependent potassium channel in adjacent Bergmann glia, and voltage-gated KVbeta2 and cyclic nucleotide-gated cation HCN1 channels in basket cells. Sedentary FVB/N and exercised C57BL/6 mice both expressed higher levels of these cation channels compared to sedentary C57BL/6 mice, and were both found to be less sensitive to glutamate toxicity. Moreover, blocking ATP-dependent potassium channels with Glibenclamide enhanced kainate-induced cell death in cerebellar slices from the resilient sedentary FVB/N mice. Furthermore, exercise increased the number of acetylcholinesterase-positive fibres in the molecular layer, reduced cerebellar cytokine levels and suppressed serum acetylcholinesterase activity, suggesting anti-inflammatory protection by enhanced cholinergic signalling. Our findings demonstrate for the first time that routine exercise and specific genetic backgrounds confer protection from cerebellar glutamatergic damages by similar molecular mechanisms, including elevated expression of cation channels. In addition, our findings highlight the involvement of the cholinergic anti-inflammatory pathway in insult-inducible cerebellar processes. These mechanisms are likely to play similar roles in other brain regions and injuries as well, opening new venues for targeted research efforts.

Polymorphisms in Ly6 genes in Msq1 encoding susceptibility to mouse adenovirus type 1

Strain-specific differences in susceptibility to mouse adenovirus type 1 (MAV-1) are linked to the quantitative trait locus Msq1 on mouse chromosome 15. This region contains 14 Ly6 or Ly6-related genes, many of which are known to be expressed on the surface of immune cells, suggesting a possible role in host defense. We analyzed these genes for polymorphisms between MAV-1-susceptible and MAV-1-resistant inbred mouse strains. Sequencing of cDNAs identified 12 coding-region polymorphisms in 2010109I03Rik, Ly6e, Ly6a, Ly6c1, and Ly6c2, six of which were nonsynonymous and five of which were previously unlisted in dbSNP Build 132. We also clarified sequence discrepancies in GenBank for the coding regions of I830127L07Rik and Ly6g. Additionally, Southern blotting revealed size polymorphisms within the DNA regions of Ly6e, Ly6a, and Ly6g. Collectively, these genetic variations have implications for the structure, function, and/or expression of Ly6 and Ly6-related genes that may contribute to the observed strain-specific differences in susceptibility to MAV-1.

The relevance of individual genetic background and its role in animal models of epilepsy

Growing evidence has indicated that genetic factors contribute to the etiology of seizure disorders. Most epilepsies are multifactorial, involving a combination of additive and epistatic genetic variables. However, the genetic factors underlying epilepsy have remained unclear, partially due to epilepsy being a clinically and genetically heterogeneous syndrome. Similar to the human situation, genetic background also plays an important role in modulating both seizure susceptibility and its neuropathological consequences in animal models of epilepsy, which has too often been ignored or not been paid enough attention to in published studies. Genetic homogeneity within inbred strains and their general amenability to genetic manipulation have made them an ideal resource for dissecting the physiological function(s) of individual genes. However, the inbreeding that makes inbred mice so useful also results in genetic divergence between them. This genetic divergence is often unaccounted for but may be a confounding factor when comparing studies that have utilized distinct inbred strains. The purpose of this review is to discuss the effects of genetic background strain on epilepsy phenotypes of mice, to remind researchers that the background genetics of a knockout strain can have a profound influence on any observed phenotype, and outline the means by which to overcome potential genetic background effects in experimental models of epilepsy.

Strain differences in seizure-induced cell death following pilocarpine-induced status epilepticus

Mouse strains differ from one another in their susceptibility to seizure-induced excitotoxic cell death. Previously, we have demonstrated that mature inbred strains of mice show remarkable genetic differences in susceptibility to the neuropathological consequences of seizures in the kainate model of status epilepticus. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. However, it remains unclear whether strain differences in susceptibility to seizure-induced cell death observed following kainate administration are observed following systemic administration of other chemoconvulsants. In rodents, the cholinomimetic convulsant pilocarpine is widely used to induce status epilepticus (SE), followed by hippocampal damage and spontaneous recurrent seizures, resembling temporal lobe epilepsy. This model has initially been described in rats, but is increasingly used in mice. We characterized neuronal pathologies after pilocarpine-induced status epilepticus (SE) in eight inbred strains of mice focusing on the hippocampus. A ramping-up dose protocol for pilocarpine was used and behavior was monitored for 4-5 h. While we did not observe any significant differences in seizure latency or duration to pilocarpine among the inbred strains, we did observe a significant difference in susceptibility to the neuropathological consequences of pilocarpine-induced SE. Of the eight genetically diverse mouse strains screened for pilocarpine-induced status, BALB/cJ and BALB/cByJ were the only two strains that were resistant to the neuropathological consequences of seizure-induced cell death. Additional studies of these murine strains may be useful for investigating genetic influences on pilocarpine-induced status epilepticus.

Congenic strains provide evidence that a mapped locus on chromosome 15 influences excitotoxic cell death

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation is unknown. Prior studies with crosses of the FVB/NJ (seizure-induced cell death susceptible) mouse and the seizure-induced cell death resistant mouse, C57BL/6J, showed the presence of three quantitative trait loci (QTLs), named seizure-induced cell death 1 (Sicd1) to Sicd3. To better localize and characterize the Sicd2 locus, two reciprocal congenic mouse strains were created. While the B6.FVB-Sicd2 congenic mouse was without effect on modifying susceptibility to seizure-induced excitotoxic cell death, the FVB.B6-Sicd2 congenic mouse, in which the chromosome (Chr) 15 region of C57BL/6J was introgressed into FVB/NJ, showed reduced seizure-induced excitotoxic cell death following kainate administration. Phenotypic comparison between FVB and the congenic FVB.B6-Sicd2 strain confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced excitotoxic cell death. Interval-specific congenic lines (ISCLs) that encompass Sicd2 on Chr 15 were generated and were used to fine-map this QTL. Resultant progeny were treated with kainate and examined for the extent of seizure-induced cell death in order to deduce the Sicd2 genotypes of the recombinants through linkage analysis. All of the ISCLs exhibited reduced cell death associated with the C57BL/6J phenotype; however, ISCL-2 showed the most dramatic reduction in seizure-induced cell death in both area CA3 and in the dentate hilus. These findings confirm the existence of polymorphic loci within the reduced critical region of Sicd2 that regulate the severity of seizure-induced cell death.

Galanin receptor 1 deletion exacerbates hippocampal neuronal loss after systemic kainate administration in mice

Background

Galanin is a neuropeptide with a wide distribution in the central and peripheral nervous systems and whose physiological effects are mediated through three G protein-coupled receptor subtypes, GalR1, GalR2, and GalR3. Several lines of evidence indicate that galanin, as well as activation of the GalR1 receptor, is a potent and effective modulator of neuronal excitability in the hippocampus.

Methodology/Principal Findings

In order to test more formally the potential influence of GalR1 on seizure-induced excitotoxic cell death, we conducted functional complementation tests in which transgenic mice that exhibit decreased expression of the GalR1 candidate mRNA underwent kainate-induced status epilepticus to determine if the quantitative trait of susceptibility to seizure-induced cell death is determined by the activity of GalR1. In the present study, we report that reduction of GalR1 mRNA via null mutation or injection of the GalR1 antagonist, galantide, prior to kainate-induced status epilepticus induces hippocampal damage in a mouse strain known to be highly resistant to kainate-induced neuronal injury. Wild-type and GalR1 knockout mice were subjected to systemic kainate administration. Seven days later, Nissl and NeuN immune- staining demonstrated that hippocampal cell death was significantly increased in GalR1 knockout strains and in animals injected with the GalR1 antagonist. Compared to GalR1-expressing mice, GalR1-deficient mice had significantly larger hippocampal lesions after status epilepticus.

Conclusions/Significance

Our results suggest that a reduction of GalR1 expression in the C57BL/6J mouse strain renders them susceptible to excitotoxic injury following systemic kainate administration. From these results, GalR1 protein emerges as a new molecular target that may have a potential therapeutic value in modulating seizure-induced cell death.

Neuroprotection by glutamate receptor antagonists against seizure-induced excitotoxic cell death in the aging brain

We previously have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death among two inbred strains of mice. We have also reported that the age-related increased susceptibility to the neurotoxic effects of seizure-induced injury is regulated in a strain-dependent manner. In the present study, we wanted to begin to determine the pharmacological mechanism that contributes to variability in the response to the neurotoxic effects of kainate. Thus, we compared the effects of the NMDA receptor antagonist, MK-801 and of the AMPA receptor antagonist NBQX on hippocampal damage in the kainate model of seizure-induced excitotoxic cell death in young, middle-aged, and aged C57BL/6 and FVB/N mice, when given 90 min following kainate-induced status epilepticus. Following kainate injections, mice were scored for seizure activity and brains from mice in each age and antagonist group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced injury. Administration of MK-801 significantly reduced the extent of hippocampal damage in young, mature and aged FVB/N mice, while application of NBQX was only effective at attenuating cell death in young and aged mice throughout all hippocampal subfields. Our results suggest that both NMDA and non-NMDA receptors are involved in kainate-induced cell death in the mouse and suggest that aging may differentially affect the ability of neuroprotectants to protect against hippocampal damage. Differences in the effectiveness of these two antagonists could result from differential regulation of glutamatergic neurotransmitter systems or ion channel specificity.

Hippocampal damage after intra-amygdala kainic acid-induced status epilepticus and seizure preconditioning-mediated neuroprotection in SJL mice

Exposure of the brain to a stressful stimulus that is sub-threshold for permanent injury can temporarily protect against cell death during a subsequent and otherwise damaging insult. One or more brief, non-harmful seizure episode(s) (seizure preconditioning) can dramatically reduce hippocampal damage when given prior to status epilepticus (epileptic tolerance). We recently reported that status epilepticus-induced hippocampal damage in C57BL/6 mice could be reduced by approximately 50% when preceded 24h earlier by a brief, non-injurious generalized seizure induced by 15mg/kg systemic kainic acid (KA). Since other mouse strains might display different vulnerability to either seizure preconditioning or status epilepticus, we investigated whether epileptic tolerance could be acquired in another strain. SJL mice, reported to display greater seizure sensitivity to systemic KA, received intra-amygdala microinjection of KA to trigger status epilepticus. Intracerebral recordings confirmed evoked seizures involved the ipsilateral hippocampus. Status epilepticus produced hippocampal damage which mainly affected the ipsilateral CA3 and hilus; a pattern similar to C57BL/6 mice. The damage extended through the full rostro-caudal extent of the hippocampal formation. Seizure preconditioning using 20mg/kg systemic KA, but not 15mg/kg, significantly reduced hippocampal damage after status epilepticus by 37% in the dorsal hippocampus and by 65% in the ventral hippocampus. These studies suggest status epilepticus induced by intra-amygdala KA in SJL mice models aspects of the pathophysiology of human mesial temporal sclerosis. Moreover, seizure preconditioning effectively produces neuroprotection in SJL mice, further establishing epileptic tolerance as a conserved endogenous neuroprotection paradigm.

The major locus for mouse adenovirus susceptibility maps to genes of the hematopoietic cell surface-expressed LY6 family

Susceptibility to mouse adenovirus type 1 is associated with the major quantitative trait locus Msq1. Msq1 was originally mapped to a 13-Mb region of mouse chromosome (Chr) 15 in crosses between SJL/J and BALB/cJ inbred mice. We have now narrowed Msq1 to a 0.75-Mb interval from 74.68 to 75.43 Mb, defined by two anonymous markers, rs8259436 and D15Spn14, using data from 1396 backcross mice. The critical interval includes 14 Ly6 or Ly6-related genes, including Ly6a (encoding Sca-1/TAP), Ly6e (Sca-2/Tsa1), Ly6g (Gr-1), and gpihbp1 (GPI-anchored high-density lipoprotein-binding protein 1), as well as the gene encoding an aldosterone synthase (Cyp11b2). The Ly6 family members are attractive candidates for virus susceptibility genes because their products are GPI-anchored membrane proteins expressed on lymphoid and myeloid cells, with proposed functions in cell adhesion and cell signaling. To determine interstrain variation in susceptibility and produce additional resources for cloning Msq1, we assayed the susceptibility phenotype of four previously untested inbred mouse strains. Susceptibility of strain 129S6/SvEvTac was subsequently localized to the Ly6 complex region, using polymorphic genetic markers on Chr 15 in a population of 271 (129S6/SvEvTac x BALB/cJ)F(1) x BALB/cJ backcross mice. We identified a major 129S6/SvEvTac susceptibility allele, Msq1(129S6), on Chr 15 in the same region as Msq1(SJL). The results indicate that a major host factor in mouse adenovirus type 1 susceptibility is likely to be a member of the Ly6 gene family.

Fine mapping of gene regions regulating neurodegeneration

Background

Damage to nerve cells and axons leading to neurodegeneration is a characteristic feature of many neurological diseases. The degree of genetic influence on susceptibility to axotomy-induced neuronal death has so far been unknown. We have examined two gene regions, Vra1 and Vra2, previously linked to nerve cell loss after ventral root avulsion in a rat F2 intercross between the DA and PVG inbred rat strains.

Methodology/Principal Findings

In this study, we use two generations (G8 and G10 cohorts) of an advanced intercross line between DA and PVG^av1 to reproduce linkage to Vra1 and to fine-map this region. By isolating the effect from Vra1 in congenic strains, we demonstrate that Vra1 significantly regulates the loss of motoneurons after avulsion. The regulatory effect mediated by Vra1 thus resides in a congenic fragment of 9 megabases. Furthermore, we have used the advanced intercross lines to give more support to Vra2, originally detected as a suggestive QTL.

Conclusions/Significance

The results demonstrated here show that naturally occurring allelic variations affect susceptibility to axotomy-induced nerve cell death. Vra1 and Vra2 represent the first quantitative trait loci regulating this phenotype that are characterized and fine mapped in an advanced intercross line. In addition, congenic strains provide experimental evidence for the Vra1 effect on the extent of injury-induced neurodegeneration. Identification of the underlying genetic variations will increase our understanding of the regulation and mechanisms of neurodegeneration.

Neuroprotection against excitotoxic brain injury in mice after ovarian steroid depletion

Ovarian steroid hormones influence not only seizure phenomena, but also the neuronal cell death that follows. In the present study, we applied two models of ovarian steroid loss, ovariectomy and chemically-induced ovarian failure, to evaluate kainate-induced seizure activity and the susceptibility of hippocampal neurons to seizure-induced neurodegeneration. Young adult female FVB/NJ mice were ovariectomized with (OVX+E, n=6) or without (OVX, n=8) estrogen replacement. A separate group of females received the ovotoxin, 4-vinylcyclohexene diepoxide (VCD, n=8) to deplete ovarian follicles. Mice underwent kainate-induced status epilepticus and were evaluated for seizure activity (3 h) and delayed hippocampal neuronal injury (7 days). While there were no differences in latency or duration of severe seizures among control, OVX and VCD-treated mice, OVX+E mice exhibited seizures of a significantly longer duration. However, both VCD-induced ovarian failure and OVX led to a dramatic reduction in the extent of excitotoxic cell death, with slightly greater effects observed in VCD-treated mice. Estradiol administration to OVX mice also exerted a significant neuroprotective effect against kainate-induced cell death. These results support and extend earlier findings suggesting that the hormonal milieu may have differential effects on seizure susceptibility that are separate and distinct from those influencing hippocampal neuronal vulnerability. Collectively, these findings highlight the complex interactions among the loss of ovarian steroid hormones, estrogen replacement, seizures, and seizure-induced cell death.

Dissociation of seizure traits in inbred strains of mice using the flurothyl kindling model of epileptogenesis

Previous seizure models have demonstrated genetic differences in generalized seizure threshold (GST) in inbred mice, but the genetic control of epileptogenesis is relatively unexplored. The present study examined, through analysis of inbred strains of mice, whether the seizure characteristics observed in the flurothyl kindling model are under genetic control. Eight consecutive, daily generalized seizures were induced by flurothyl in mice from five inbred strains. Following a 28-day rest period, mice were retested with flurothyl. The five strains of mice demonstrated inter-strain differences in GST, decreases in GST across seizure trials, and differences in the behavioral seizure phenotypes expressed. Since many of the seizure characteristics that we examined in the flurothyl kindling model were dissociable between C57BL/6J and DBA/2J mice, we analyzed these strains in detail. Unlike C57BL/6J mice, DBA/2J mice had a lower GST on trial 1, did not demonstrate a decrease in GST across trials, nor did they show an alteration in seizure phenotype upon flurothyl retest. Surprisingly, [C57BL/6JxDBA/2J] F1-hybrids had initial GST on trial 1 and GST decreases across trials similar to what was found for C57BL/6J, but they did not undergo the alteration in behavioral seizure phenotype that had been observed for C57BL/6J mice. Our data establish the significance of the genetic background in flurothyl-induced epileptogenesis. The [C57BL/6JxDBA/2J] F1-hybrid data demonstrate that initial GST, the decrease in GST across trials, and the change in seizure phenotype differ from the characteristics of the parental strains, suggesting that these phenotypes are controlled by independent genetic loci.

Mutation-, aging-, and gene dosage-dependent accumulation of neuroserpin (G392E) in endoplasmic reticula and lysosomes of neurons in transgenic mice

Mutations in human neuroserpin gene cause an autosomal dementia, familial encephalopathy with neuroserpin inclusion bodies (FENIB). We generated and analyzed transgenic mice expressing high levels of either FENIB-type (G392E) or wild-type human neuroserpin in neurons of the central nervous system. G392E neuroserpin accumulated age-dependently in neurons of the neocortex, thalamus, amygdala, pons, and spinal cord of homozygous transgenic mice. Such accumulations were not observed in hemizygous transgenic mice nor in transgenic mice for wild-type neuroserpin. In differential centrifugation of brain homogenates, G392E neuroserpin recovered in the nucleus-rich fraction dramatically increased along with aging, suggesting that the aggregations gradually increase their densities presumably by their conversion into heavier and more compact configurations. In immunoelectron microscopical analyses, immunopositivities for G392E neuroserpin were found not only in endoplasmic reticulum but also in lysosomes. G392E neuroserpin transgenic mice were much more susceptible to seizures induced by kainate administration than nontransgenic mice. Overall, G392E neuroserpin accumulated in the central nervous system neurons of transgenic mice in mutation-, aging-, and gene dosage-dependent manners. The established transgenic mice will be valuable to elucidate not only mechanisms for the formation of G392E neuroserpin aggregations but also pathways for the degradation and/or clearance of the already formed aggregations in neurons.

Variation in Galr1 expression determines susceptibility to exocitotoxin-induced cell death in mice

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation in differential susceptibility is unknown. Previously, we identified a highly significant quantitative trait locus (QTL) on chromosome 18 that influenced susceptibility to kainic acid-induced cell death (Sicd1). Comparison of susceptibility to seizure-induced cell death between reciprocal congenic lines for Sicd1 and parental background mice indicates that genes influencing this trait were captured in both strains. Two positional gene candidates, Galr1 and Mbp, map to 55 cM, where the Sicd1 QTL had been previously mapped. Thus, this study was undertaken to determine if Galr1 and/or Mbp could be considered as candidate genes. Genomic sequence comparison of these two functional candidate genes from the C57BL/6J (resistant at Sicd1) and the FVB/NJ (susceptible at Sicd1) strains showed no single-nucleotide polymorphisms. However, expression studies confirmed that Galr1 shows significant differential expression in the congenic and parental inbred strains. Galr1 expression was downregulated in the hippocampus of C57BL/6J mice and FVB.B6-Sicd1 congenic mice when compared with FVB/NJ or B6.FVB-Sicd1 congenic mice. A survey of Galr1 expression among other inbred strains showed a significant effect such that 'susceptible' strains showed a reduction in Galr1 expression as compared with 'resistant' strains. In contrast, no differences in Mbp expression were observed. In summary, these results suggest that differential expression of Galr1 may contribute to the differences in susceptibility to seizure-induced cell death between cell death-resistant and cell death-susceptible strains.

Effect of age on kainate-induced seizure severity and cell death

While the onset and extent of epilepsy increases in the aged population, the reasons for this increased incidence remain unexplored. The present study used two inbred strains of mice (C57BL/6J and FVB/NJ) to address the genetic control of age-dependent neurodegeneration by building upon previous experiments that have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death. We determined if seizure induction and seizure-induced cell death are affected differentially in young adult, mature, and aged male C57BL/6J and FVB/NJ mice administered the excitotoxin, kainic acid. Dose response testing was performed in three to four groups of male mice from each strain. Following kainate injections, mice were scored for seizure activity and brains from mice in each age group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced brain damage. Irrespective of the dose of kainate administered or the age group examined, resistant strains of mice (C57BL/6J) continued to be resistant to seizure-induced cell death. In contrast, aged animals of the FVB/NJ strain were more vulnerable to the induction of behavioral seizures and associated neuropathology after systemic injection of kainic acid than young or middle-aged mice. Results from these studies suggest that the age-related increased susceptibility to the neurotoxic effects of seizure induction and seizure-induced injury is regulated in a strain-dependent manner, similar to previous observations in young adult mice.

Mouse models of human KCNQ2 and KCNQ3 mutations for benign familial neonatal convulsions show seizures and neuronal plasticity without synaptic reorganization

The childhood epilepsy syndrome of benign familial neonatal convulsions (BFNC) exhibits the remarkable feature of clinical remission within a few weeks of onset and a favourable prognosis, sparing cognitive abilities despite persistent expression of the mutant KCNQ2 or KCNQ3 potassium channels throughout adulthood. To better understand such dynamic neuroprotective plasticity within the developing brain, we introduced missense mutations that underlie human BFNC into the orthologous murine Kcnq2 (Kv7.2) and Kcnq3 (Kv7.3) genes. Mutant mice were examined for altered thresholds to induced seizures, spontaneous seizure characteristics, hippocampal histology, and M-current properties of CA1 hippocampal pyramidal neurons. Adult Kcnq2(A306T/+) and Kcnq3(G311V/+) heterozygous knock-in mice exhibited reduced thresholds to electrically induced seizures compared to wild-type littermate mice. Both Kcnq2(A306T/A306T) and Kcnq3(G311V/G311V) homozygous mutant mice exhibited early onset spontaneous generalized tonic-clonic seizures concurrent with a significant reduction in amplitude and increased deactivation kinetics of the neuronal M-current. Mice had recurrent seizures into adulthood that triggered molecular plasticity including ectopic neuropeptide Y (NPY) expression in granule cells, but without hippocampal mossy fibre sprouting or neuronal loss. These novel knocking mice recapitulate proconvulsant features of the human disorder yet show that inherited M-current defects spare granule cells from reactive changes in adult hippocampal networks. The absence of seizure-induced pathology found in these epileptic mouse models parallels the benign neurodevelopmental cognitive profile exhibited by the majority of BFNC patients.

A quantitative trait locus on chromosome 18 is a critical determinant of excitotoxic cell death susceptibility

C57BL/6J (B6) and FVB/NJ (FVB) mice are phenotypically distinct in their susceptibility to seizure-induced cell death after kainate administration. Previous studies using quantitative trait loci (QTLs) mapping established that the distal region of mouse chromosome 18 contains a gene(s) that is probably responsible for the difference in seizure-induced cell death susceptibility between two inbred strains, B6 and FVB, that are relatively resistant and susceptible, respectively, to seizure-induced cell death. The genetic locus has been mapped to a approximately 12-centimorgan region of chromosome 18, designated as seizure-induced cell death 1 (Sicd1). In order to confirm the Sicd1 QTL, we have developed congenic mouse strains containing the relevant donor segment from the resistant B6 strain on the susceptible FVB background, also referred to as the FVB.B6-Sicd1 congenic strain. Congenic and FVB littermate controls were tested in a seizure-induced cell death paradigm. The presence of B6 chromosome 18 alleles on an FVB genetic background conferred protection against seizure-induced cell death, as compared with FVB littermate controls. To further localize the Sicd1 QTL, new congenic lines carrying overlapping intervals of the B6 segment were created [interval-specific congenic lines (ISCLs)-1-4] and assessed for seizure-induced cell death phenotype. All of the ISCLs exhibited reduced cell death associated with the B6 phenotype, as compared with the parental FVB strain. The most dramatic of these, ISCL-4, showed a nearly four-fold reduction in the extent of seizure-induced cell death. This suggests that ISCL-4 contains the putative gene(s) of the Sicd1 QTL.

Differential susceptibility to striatal neurodegeneration induced by quinolinic acid and kainate in inbred, outbred and hybrid mouse strains

In mice, the genetic background determines susceptibility to hippocampal neurodegeneration induced by the excitotoxin kainic acid (KA). If genetic background plays as significant a role in the striatum, the area most affected in Huntington's disease (HD), it is important to characterize intrinsic differences in neuronal susceptibility in mouse strains used in HD models. This study was performed to investigate whether strain differences of different HD mouse models are determinants of striatal resistance to excitotoxicity. We conducted a survey of the susceptibility of striatal neurons to neurodegeneration induced by quinolinic acid and KA in six inbred, two outbred and two F1 hybrid (resistant*vulnerable) strains. These are the same strains in which we have assessed vulnerability to KA-induced hippocampal neurodegeneration. We found significant strain differences in response to both excitotoxins and, for the most part, the strain-dependent patterns of susceptibility to quinolinic acid and KA were similar and comparable to those previously found with KA-induced hippocampal neurodegeneration. There were some incongruities, suggesting that the genetic determinants may be different for the two forms of excitotoxicity or that there are important interacting factors. For example, the F1 hybrid strains exhibited neurodegeneration similar to their vulnerable parent, indicating that the vulnerable phenotype is dominant. This is in contrast to KA-induced hippocampal neurodegeneration, where F1 hybrids exhibit the resistant phenotype. These results are also of significance with regard to the issue of region-specific vulnerability in the context of different diseases in which genetic modifiers affect age of onset and/or disease progression.

Use of chromosome substitution strains to identify seizure susceptibility loci in mice

Seizure susceptibility varies among inbred mouse strains. Chromosome substitution strains (CSS), in which a single chromosome from one inbred strain (donor) has been transferred onto a second strain (host) by repeated backcrossing, may be used to identify quantitative trait loci (QTLs) that contribute to seizure susceptibility. QTLs for susceptibility to pilocarpine-induced seizures, a model of temporal lobe epilepsy, have not been reported, and CSS have not previously been used to localize seizure susceptibility genes. We report QTLs identified using a B6 (host) x A/J (donor) CSS panel to localize genes involved in susceptibility to pilocarpine-induced seizures. Three hundred fifty-five adult male CSS mice, 58 B6, and 39 A/J were tested for susceptibility to pilocarpine-induced seizures. Highest stage reached and latency to each stage were recorded for all mice. B6 mice were resistant to seizures and slower to reach stages compared to A/J mice. The CSS for Chromosomes 10 and 18 progressed to the most severe stages, diverging dramatically from the B6 phenotype. Latencies to stages were also significantly shorter for CSS10 and CSS18 mice. CSS mapping suggests seizure susceptibility loci on mouse Chromosomes 10 and 18. This approach provides a framework for identifying potentially novel homologous candidate genes for human temporal lobe epilepsy.

Comparison of seizure phenotype and neurodegeneration induced by systemic kainic acid in inbred, outbred, and hybrid mouse strains

We assessed inbred, outbred and hybrid mouse strains for susceptibility to seizures and neurodegeneration induced by systemic administration of kainic acid (KA). Each strain showed a unique pattern of susceptibility to seizures as assessed by the dose necessary to induce continuous tonic clonic seizures, progression through six seizure levels, the number of mice that failed to satisfy seizure criteria, and seizure-induced mortality. In general, the C57BL/6, ICR, FVB/N, and BALB/c strains were resistant to seizures while the C57BL/10, DBA/2 J, and F1 C57BL/6*CBA/J strains were vulnerable. Neuronal cell death was quantified in four subfields of the hippocampus: CA3, the hilus of the dentate gyrus, CA1, and the dentate granule cell layer. Neurodegeneration was also semiquantitatively assessed in other brain regions including the neocortex, striatum, thalamus, hypothalamus and amygdala. Although there was variability in the extent of cell death within strains, there were significant differences in the amount of hippocampal cell death between strains and also different patterns of neurodegeneration in affected brain areas. In general, the C57BL/6, C57BL/10, and F1 C57BL/6*CBA/J strains were resistant to neurodegeneration while the FVB/N, ICR and DBA/2 J strains were vulnerable. The BALB/c strain was unique in that neurodegeneration was confined to the hippocampus. Consistent with previous findings, the resistant neurodegeneration phenotype was dominant in an F1 cross of resistant and vulnerable inbred strains. Our results, using a large number of mouse strains, definitively demonstrate that a mouse strain's seizure phenotype is not related to its neurodegeneration phenotype.

Genetic modifiers affecting severity of epilepsy caused by mutation of sodium channelScn2a

Mutations in the voltage-gated sodium channels SCN 1 A and SCN 2 A are responsible for several types of human epilepsy. Variable expressivity among family members is a common feature of these inherited epilepsies, suggesting that genetic modifiers may influence the clinical manifestation of epilepsy. The transgenic mouse model Scn 2 a(Q 54) has an epilepsy phenotype as a result of a mutation in Scn 2 a that slows channel inactivation. The mice display progressive epilepsy that begins with short-duration partial seizures that appear to originate in the hippocampus. The partial seizures become more frequent and of longer duration with age and often induce secondary generalized seizures. Clinical severity of the Scn 2 a(Q 54) phenotype is influenced by genetic background. Congenic C57BL/6J.Q 54 mice exhibit decreased incidence of spontaneous seizures, delayed seizure onset, and longer survival in comparison with [C57BL/6J x SJL/J]F(1).Q 54 mice. This observation indicates that strain SJL/J carries dominant modifier alleles at one or more loci that determine the severity of the epilepsy phenotype. Genome-wide interval mapping in an N(2) backcross revealed two modifier loci on Chromosomes 11 and 19 that influence the clinical severity of of this sodium channel-induced epilepsy. Modifier genes affecting clinical severity in the Scn 2 a(Q 54) mouse model may contribute to the variable expressivity seen in epilepsy patients with sodium channel mutations.

Sodium channel mutations in epilepsy and other neurological disorders

Since the first mutations of the neuronal sodium channel SCN1A were identified 5 years ago, more than 150 mutations have been described in patients with epilepsy. Many are sporadic mutations and cause loss of function, which demonstrates haploinsufficiency of SCN1A. Mutations resulting in persistent sodium current are also common. Coding variants of SCN2A, SCN8A, and SCN9A have also been identified in patients with seizures, ataxia, and sensitivity to pain, respectively. The rapid pace of discoveries suggests that sodium channel mutations are significant factors in the etiology of neurological disease and may contribute to psychiatric disorders as well.

Identification of quantitative trait loci for susceptibility to mouse adenovirus type 1

Adult SJL/J mice are highly susceptible to mouse adenovirus type 1 (MAV-1) infections, whereas other inbred strains, including BALB/cJ, are resistant (K. R. Spindler, L. Fang, M. L. Moore, C. C. Brown, G. N. Hirsch, and A. K. Kajon, J. Virol. 75:12039-12046, 2001). Using congenic mouse strains, we showed that the H-2(s) haplotype of SJL/J mice is not associated with susceptibility to MAV-1. Susceptibility of MAV-1-infected (BALB/cJ x SJL/J)F(1) mice was intermediate between that of SJL/J mice and that of BALB/cJ mice, indicating that susceptibility is a genetically controlled quantitative trait. We mapped genetic loci involved in mouse susceptibility to MAV-1 by analysis of 192 backcross progeny in a genome scan with 65 simple sequence length polymorphic markers. A major quantitative trait locus (QTL) was detected on chromosome 15 (Chr 15) with a highly significant logarithm of odds score of 21. The locus on Chr 15 alone accounts for 40% of the total trait variance between susceptible and resistant strains. QTL modeling of the data indicated that there are a number of other QTLs with small effects that together with the major QTL on Chr 15 account for 54% of the trait variance. Identification of the major QTL is the first step in characterizing host genes involved in susceptibility to MAV-1.

Effects of seizures on brain development: lessons from the laboratory

Both clinical and laboratory studies demonstrate that seizures early in life can result in permanent behavioral abnormalities and enhance epileptogenicity. In experimental rodent models, the consequences of seizures are dependent upon age, etiology, seizure duration, and frequency. Recurrent seizures in immature rats result in long-term adverse effects on learning and memory. These behavioral changes are paralleled by changes in brain connectivity, dendritic morphology, excitatory and inhibitory receptor subunits, ion channels, and neurogenesis. These changes can occur in the absence of cell loss. Although impaired cognitive function and brain changes have been well documented after early onset seizures, the mechanisms of seizure-induced injury remain unclear. Recent studies have demonstrated abnormalities in single cell function that parallel behavioral changes.